Touch device and signal processing circuit as well as operating method thereof

ABSTRACT

A signal processing circuit of a touch device including an operational amplifier, a feedback resistor and a step current circuit is provided. The feedback resistor connects between a negative input and an output terminal of the operational amplifier. The step current circuit is coupled to the negative input of the operational amplifier and configured to provide or draw a step current to reduce the current flowing through the feedback resistor so as to compensate the voltage offset of the operational amplifier.

BACKGROUND

1. Field of the Disclosure

This disclosure generally relates to a touch device, more particularly,to a touch device and a signal processing circuit as well as anoperating method thereof that may reduce the offset error caused by theoffset voltage of an operational amplifier to increase the signaldynamic range.

2. Description of the Related Art

Touch device or touch panel is used to detect touch signal resultingfrom a touch event or press event. The detection signals outputted bythe touch panel are successively processed by an analog front end and adigital back end to accordingly identify whether the touch panel isoperated by a user.

The analog front end may adopt an integrated programmable gain amplifier(IPGA) as shown in FIG. 1 to amplify the detection signals. The IPGAincludes an operational amplifier which has an offset between two inputterminals due to the manufacturing process, and this offset causes anoffset voltage which can reduce a signal dynamic range of the digitalback end connected downstream such that the signal-to-noise ratio (SNR)and the operating performance of the touch panel are degraded.

Accordingly, the present disclosure provides a touch device and a signalprocessing circuit as well as an operating method thereof that mayeliminate or at least significantly alleviate the signal dynamic rangedecrease caused by the offset voltage of the operational amplifier.

SUMMARY

The present disclosure provides a touch device and a signal processingcircuit as well as an operating method thereof that may reduce theoccupied area of a compensation capacitor in the IPGA by employing anattenuation circuit.

The present disclosure further provides a touch device and a signalprocessing circuit as well as an operating method thereof that mayalleviate the influence of the offset voltage of an operationalamplifier by employing a step current circuit to accordingly maintainthe signal dynamic range.

The present disclosure provides a signal processing circuit of a touchdevice including an integrated programmable gain amplifier (IPGA), anattenuation circuit and a step current circuit. The IPGA includes afirst operational amplifier, a feedback resistor, a compensationcapacitor and an input resistor. The first operational amplifier has apositive input, a negative input and an output terminal. The feedbackresistor connects between the negative input and the output terminal ofthe first operational amplifier. The compensation capacitor connectsbetween the negative input and the output terminal of the firstoperational amplifier. A first terminal of the input resistor is coupledto the negative input of the first operational amplifier. Theattenuation circuit is coupled to a second terminal of the inputresistor to split a current flowing through the input resistor. The stepcurrent circuit is coupled to the negative input of the firstoperational amplifier, and configured to provide a step current to theIPGA or draw a step current from the IPGA.

The present disclosure further provides a touch device including acapacitive touch panel and a control chip. The capacitive touch panelincludes a plurality of sense electrodes each configured to output adetection signal. The control chip includes a plurality of integratedprogrammable gain amplifiers (IPGAs), a plurality of attenuationcircuits and a plurality of step current circuits. The IPGAs arerespectively coupled to the plurality of sense electrodes to amplify thedetection signal. Each of the IPGAs includes a first operationalamplifier, a feedback resistor, a compensation capacitor and an inputresistor. The first operational amplifier has a positive input, anegative input and an output terminal. The feedback resistor connectsbetween the negative input and the output terminal of the firstoperational amplifier. The compensation capacitor connects between thenegative input and the output terminal of the first operationalamplifier. A first terminal of the input resistor is coupled to thenegative input of the first operational amplifier. Each of theattenuation circuits is coupled to a second terminal of the inputresistor of one of the IPGAs, and configured to split a current flowingthrough the input resistor. Each of the step current circuits is coupledto the negative input of the first operational amplifier of one of theIPGAs, and configured to provide a step current to the coupled IPGA ordraw a step current from the coupled IPGA.

The present disclosure further provides an operating method of a touchdevice. The touch device includes a capacitive touch panel, anintegrated programmable gain amplifier (IPGA), an attenuation circuit, astep current circuit, an analog-to-digital converter and a digital backend. The capacitive touch panel, the attenuation circuit, the stepcurrent circuit and the analog-to-digital converter are coupled to theIPGA. The attenuation circuit attenuates a gain of the IPGA. Theanalog-to-digital converter is coupled between the IPGA and the digitalback end. The operating method includes the steps of: entering acalibration stage; receiving, by the IPGA, a detection signal outputtedby the capacitive touch panel to output an amplified detection signal;converting, by the analog-to-digital converter, the amplified detectionsignal to a digital signal; and controlling, by the digital back end,the step current circuit to provide a step current to the IPGA or draw astep current from the IPGA according to the digital signal to reduce anoffset current value caused by an offset voltage difference between theIPGA and the attenuation circuit.

The step current circuit of the present disclosure is a current mirrorcircuit. The current mirror circuit provides a step current to anintegrated programmable gain amplifier (IPGA) or draws a step currentfrom the IPGA to reduce the current flowing through a feedback resistorof an operational amplifier. Therefore, it is possible to eliminate orreduce at least a part of the signal dynamic range decrease caused bythe offset voltage of the operational amplifier.

The attenuation circuit of the present disclosure is connected to theIPGA for reducing a gain thereof so as to prevent the output saturationof the IPGA. Accordingly, it is not necessary to reduce the gain of theIPGA by increasing a compensation capacitor of the IPGA such that theoccupied area of the compensation capacitor in the control chip issignificantly reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages, and novel features of the present disclosurewill become more apparent from the following detailed description whentaken in conjunction with the accompanying drawings.

FIG. 1 is a schematic block diagram of a conventional signal processingcircuit of a touch panel.

FIG. 2 is a schematic block diagram of a touch device according to oneembodiment of the present disclosure.

FIG. 3 is a schematic block diagram of a signal processing circuit of atouch device according to one embodiment of the present disclosure.

FIG. 4 is a schematic block diagram of a signal processing circuit of atouch device according to another embodiment of the present disclosure.

FIG. 5 is a schematic block diagram of a signal processing circuit of atouch device according to an alternative embodiment of the presentdisclosure.

FIG. 6 is a flow chart of an operating method of a touch deviceaccording to one embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENT

It should be noted that, wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

Referring to FIG. 2, it is a schematic block diagram of a touch deviceaccording to one embodiment of the present disclosure. The touch deviceincludes a capacitive touch panel 21 and a control chip 23. The touchdevice can be used to detect capacitance variance from a conductiveobject closing the capacitive touch panel 21 or an external forceapplied to the capacitive touch panel 21. The control chip 23 includes asignal processing circuit for processing detection signals y(t)outputted by the capacitive touch panel 21, wherein the detectionsignals y(t) are, for example, alternating current (ac) voltage signals.The control chip 23 is electrically connected to the capacitive touchpanel 21 via, for example, at least one bus line to communicate thedetection signals y(t) and control signals therebetween.

The capacitive touch panel 21 includes, for example, a plurality ofdrive electrodes 211 and a plurality of sense electrodes 213. Forexample, the plurality of sense electrodes 213 intersects the pluralityof drive electrodes 211 to form mutual capacitors Cm therebetween. Theplurality of drive electrodes 211 is used to receive drive signals Vsig,and the plurality of sense electrodes 213 is used to output thedetection signals y(t) according to the induction of the mutualcapacitors Cm. When a conductor (e.g., a finger or a stylus) approachesor touches the capacitive touch panel 21, the capacitance of the mutualcapacitors Cm is changed to influence the detection signals y(t), andthus the control chip 23 is able to identify a touch position, gestureor the like according the variation of the detection signals y(t) toperform corresponding controls, wherein said corresponding controlsperformed by the control chip 23 is determined according to differentapplications without particular limitations, such as controlling acursor, activating application software, moving an icon and so on. Themethod that the control chip 23 identifies the touch control on thecapacitive touch panel 21 according to the mutual capacitance variationis known to the art, e.g., identifying the voltage signal variation orcharging/discharging time interval variation, and thus details thereofare not described herein.

It should be mentioned that although FIG. 2 shows the plurality of driveelectrodes 211 extending along a transverse direction and the pluralityof sense electrodes 213 extending along a longitudinal direction, it isonly intended to illustrate but not to limit the present disclosure. Thespatial relationship between the plurality of drive electrodes 211 andthe plurality of sense electrodes 213 is determined according todifferent arrangements as long as the mutual capacitors Cm are formedbetween the plurality of drive electrodes 211 and the plurality of senseelectrodes 213.

In addition, the detection signals y(t) are not limited to be generatedaccording to the induction of the mutual capacitors Cm, and it ispossible that the detection signals y(t) are generated according to theinduction of the self-capacitance of the plurality of drive electrodes211 and/or the plurality of sense electrodes 213. Operations of theself-capacitance mode and the mutual capacitance mode may be referred toU.S. patent application Ser. No. 14/697,923, filed on Apr. 28, 2015,assigned to the same Assignee of the present application, and the fulldisclosure of which is incorporated herein by reference.

The control chip 23 includes a plurality of drive circuits 230 fordriving the capacitive touch panel 21 and a signal processing circuitfor processing the detection signals y(t) outputted by the capacitivetouch panel 21. The plurality of drive circuits 230 is, for example,signal generators. Each of the plurality of drive circuits 230 isrespectively coupled to one of the plurality of drive electrodes 211 todrive the coupled drive electrode 211 using a drive signal Vsig. In someembodiments, each of the plurality of drive circuits 230 is coupled tothe corresponding drive electrode 211 via a switching element (notshown), and the switching element is used to control whether the drivesignal Vsig is inputted into the coupled drive electrode 211 or not. Forexample, the drive signal Vsig is inputted into the capacitive touchpanel 21 in a normal stage, but is not inputted into the capacitivetouch panel 21 in a calibration stage.

The signal processing circuit herein is referred to the circuit coupleddownstream of the capacitive touch panel 21. The signal processingcircuit includes, for example, a plurality of integrated programmablegain amplifiers (IPGAs) 231, a plurality of attenuation circuits(described later), an analog-to-digital converter (ADC) 235 and adigital back end 237, wherein the plurality of IPGAs 231 is coupled tothe ADC 235 via, for example, a plurality of switching elements or amultiplexer. The plurality of IPGAs 231 is respectively coupled to theplurality of sense electrodes 213 (e.g., via a plurality of switchingelements) to amplify the detection signals y(t). In the presentdisclosure, as each of the plurality of attenuation circuits is coupledto one IPGA 231, it is possible to consider that each of the attenuationcircuits is included in one of the IPGAs 231.

Please referring to FIG. 3, it is a schematic block diagram of a signalprocessing circuit of a touch device according to one embodiment of thepresent disclosure, wherein FIG. 3 shows only one of the plurality ofIPGAs 231 in FIG. 2 and the corresponding mutual capacitor Cm and drivecircuit 230 thereof. Other IPGAs 231 and the corresponding mutualcapacitor Cm and drive circuit 230 are the same. Each of the IPGAs 231includes a first operational amplifier OP1, a feedback resistor Rf, acompensation capacitor Cf and an input resistor Ri, and is used toreceive and process the detection signal y(t) outputted by the coupledsense electrode 213.

The first operational amplifier OP1 has a positive input (+), a negativeinput (−) and an output terminal. The positive input is coupled to aconstant voltage source of 0.9 volt, but not limited thereto. The outputterminal is used to output an amplified detection signal Sout to the ADC235.

The ADC 235 is coupled to the output terminal of the first operationalamplifier OP1, and used to convert the amplified detection signal Soutto a digital signal. The digital back end 237 is coupled to the ADC 235,and used to control on/off of a plurality of switching devices accordingto the digital signal (described below with an example). The ADC 235 maybe included in the analog front end or the digital back end withoutparticular limitations.

The feedback resistor Rf is connected between the negative input and theoutput terminal of the first operational amplifier OP1. The compensationcapacitor Cf is also connected between the negative input and the outputterminal of the first operational amplifier OP1 to form a parallelconnection with the feedback resistor Rf. A first terminal (e.g., theright end in FIG. 3) of the input resistor Ri is coupled to the negativeinput of the first operational amplifier OP1, and a second terminal(e.g., the left end in FIG. 3) of the input resistor Ri is coupled toone of the plurality of sense electrodes 213 for receiving the detectionsignal y(t) outputted therefrom.

The attenuation circuit 233 is coupled to the second terminal of theinput resistor Ri for splitting a current flowing through the inputresistor Ri (e.g., a current Iatt being shown in FIG. 3) to attenuate again of the IPGA 231.

It is known that in order to prevent the output saturation of the IPGAshown in FIG. 1, it is possible to reduce the gain thereof by increasingthe capacitance of the compensation capacitor Cf. However, a largercompensation capacitor Cf can occupy a larger area in the control chip23. To maintain or reduce the capacitance of the compensation capacitorCf, in the present disclosure the attenuation circuit 233 is employed toreduce the gain of the IPGA 231.

In the present disclosure, the attenuation circuit 233 may use asuitable circuit without particular limitations as long as the purposeof reducing the gain of the IPGA 231 is achievable.

For example referring to FIG. 4, it is a schematic block diagram of asignal processing circuit of a touch device according to anotherembodiment of the present disclosure. In this embodiment, theattenuation circuit 233 includes a second operational amplifier OP2 anda shunt resistor Rs. The second operational amplifier OP2 has a positiveinput (+), a negative input (−) and an output terminal. The outputterminal of the second operational amplifier OP2 is directly coupled tothe negative input of the second operational amplifier OP2 to form avoltage follower. The shut resistor Rs has a first terminal (e.g., theright end in FIG. 4) coupled to the output terminal of the secondoperational amplifier OP2, and a second terminal (e.g., the left end inFIG. 4) coupled to the second terminal of the input resistor Ri tocouple to one of the plurality of sense electrodes 213 together with theinput resistor Ri.

In some circumstances, due to the manufacturing process, it is possiblethat the operational amplifiers OP1 and OP2 have different features, andthe offset of individual operational amplifiers OP1 and OP2 may furthercause an offset existing between the first operational amplifier OP1 ofthe IPGA 231 and the attenuation circuit 233 due to the mismatchtherebetween. Therefore, an offset voltage exists between theoperational amplifiers OP1 and OP2, e.g., a voltage difference betweennodes V_(X) and V_(Y). This offset voltage difference can generate anoffset current flowing through the feedback resistor Rf to cause anoffset of the amplified detection signal Sout such that an operabledynamic range of the ADC 235 is reduced. More specifically, although itis able to reduce the capacitance of the compensation capacitor Cf bydisposing the attenuation circuit 233, the offset current caused by theoffset voltage difference between the IPGA 231 and the attenuationcircuit 233 is generated to flow through the input resistor Ri and thefeedback resistor Rf.

For example, if it is assumed that the feedback resistor Rf is 100 Kohm,the input resistor Ri is 32 Kohm and the shunt resistor Rs is 1 Kom, adc gain Rf/(Ri+Rs) is calculated to be about 3. If it is further assumedthat the offset voltages of the first operational amplifier OP1 and thesecond operational amplifier OP2 are both 30 mV, a maximum possibleoffset voltage difference between the first operational amplifier OP1and the second operational amplifier OP2 is up to 60 mV. After theamplification of the IPGA 231, the maximum possible offset voltage canreach 60×3=180 mV. If an operable dynamic range of the ADC 235 is 400mV, the 180 mV offset voltage is 45% of the operable dynamic range. Itis clear that the operating performance of the capacitive touch panel 21can be significantly degraded. In addition, to maintain the cutofffrequency 1/(2π×Rf×Cf) of the IPGA 231, in some embodiments a value ofthe feedback resistor Rf is increased such that the influence caused bythe offset voltage is further enhanced.

The present disclosure further provides a signal processing circuitcapable of eliminating or significantly reducing the offset voltagementioned above.

Referring to FIG. 5, it is a schematic block diagram of a signalprocessing circuit of a touch device according to an alternativeembodiment of the present disclosure. In this embodiment, the signalprocessing circuit further includes a plurality of step current circuits239. Each of the plurality of step current circuits 239 is respectivelycoupled to the negative input of the first operational amplifier OP1 ofone of the IPGAs 231, and used to provide a step current to the coupledIPGA 231 or draw a step current from the coupled IPGA 231, wherein thestep current herein is referred to that the current provided or drawn bythe step current circuits 239 is not changed continuously but changed bya fixed value in a step-by-step manner. Similarly, in this embodiment,as each of the plurality of step current circuits 239 is coupled to oneof the IPGAs 231, it is possible to consider that each of the pluralityof step current circuits 239 is a part of one IPGA 231.

In one embodiment, the step current circuit 239 includes a currentmirror circuit. The current mirror circuit includes a plurality ofswitching devices (e.g., 6 switching devices 0 to 5 being shown in FIG.5 such as PMOS switching devices and/or NMOS switching devices). Thecurrent mirror circuit is controlled by the digital back end 237, andthe current flowing through the feedback resistor Rf is controlled tohave a smallest value by turning on/off the plurality of switchingdevices (described below).

For example, the operation of the touch device of the present disclosureincludes a calibration stage and a normal stage, wherein the calibrationstage is referred to a stage of a starting procedure, ending a sleepmode or receiving a command during which there is no conductorapproaching or touching the touch device and the touch detection is notbeing performed, while the normal stage is referred to a stage duringwhich the touch device is performing the touch detection. The two stagesare distinguishable, for example, by identifying whether the drivesignal Vsig is inputted or activated.

For example, in the calibration stage, the plurality of drive circuits230 of the control chip 23 does not input the drive signals Vsig to thecorresponding drive electrodes 211. Meanwhile, the digital back end 237previously stores predetermined digital values (e.g., stored beforeshipment) associated with the drive signal Vsig not being used to drivethe drive electrodes 211. When the above mentioned offset voltageexists, the digital value formed by the ADC 235 converting the amplifieddetection signal Sout and being inputted to the digital back end 237 isdifferent from (larger or smaller than) the predetermined digital value.

The digital back end 237 controls multiple switching devices in the stepcurrent circuit 239, e.g., sending a control signal VBP to controlswitching devices 0 to 2 to cause the voltage source VDD to provide astep current Ip to the corresponding IPGA 231, or sending a controlsignal VBN to control switching devices 3 to 5 to draw a step current Idfrom the corresponding IPGA 231 to the ground GRD. That is, the stepcurrent circuit 239 includes a current source and a current sink. Thegoal of the control is to obtain a control code (e.g., having apredetermined bit) of the multiple switching devices allowing thecurrent flowing through the feedback resistor Rf to have a smallestvalue, i.e. the digital value outputted by the ADC 235 being close to oreven equal to the predetermined digital value. Meanwhile, the obtainedcontrol code is stored in a storage device of the digital back end 237,e.g., storing in a volatile memory such as a random access memory (RAM)or a flash memory. The stored control code is for the digital back end237 to control the on/off of the plurality of switching devices (e.g.,switching devices 0 to 5) in the normal stage.

Following the above example, when the offset voltage difference betweenthe first and second operational amplifiers OP1 and OP2 is 60 mV, theoffset current is calculated as 60 mV/(32 Kohm+1 Kohm)=1.818 μAmp. If a3-bits calibration is used, 7 different step currents are usable suchthat the calibration resolution is calculated as 1.818 μAmp/(7×2)=0.13μAmp. The output offset voltage of the IPGA 231 is then calculated as0.13 μAmp×100 Kohm=13 mV, which is significantly lower than the aboveuncalibrated value.

It is appreciated that as the manufacturing factor has different effectson each of the IPGAs 231 and the attenuation circuits 233, the digitalback end 237 preferably pre-stores one control code corresponding toeach of the plurality of IPGAs 231 such that the offset voltage of theamplified detection signal Sout outputted by each of the IPGAs 231 has asmallest value to improve the detection accuracy. In addition, thecalibration resolution of the step current circuit 239 is determined bya number of the switching devices. If more switching devices are used,more accurate calibration is obtainable to obtain a smaller offsetvoltage. Therefore, a number of the switching devices actually beingused is determined according to actual design without particularlimitations.

Please referring to FIG. 6, it is a flow chart of an operating method ofa touch device according to one embodiment of the present disclosure.The operating method includes the steps of: entering a calibration stage(Step S61); receiving, by an IPGA, a detection signal outputted by acapacitive touch panel to output an amplified detection signal (StepS63); converting, by an ADC, the amplified detection signal to a digitalsignal (Step S65); controlling, by a digital back end, the step currentcircuit to provide a step current to the IPGA or draw a step currentfrom the IPGA according to the digital signal to reduce an offsetcurrent value caused by an offset voltage difference between the IPGAand the attenuation circuit (Step S67); and entering a normal stage, andcontrolling the step current circuit according to previously storedcontrol codes (Step S69).

The operating method of this embodiment is adaptable to a touch devicewhich includes the signal processing circuit of FIG. 5. As mentionedabove, the touch device includes a capacitive touch panel 21, anintegrated programmable gain amplifier (IPGA) 231, an attenuationcircuit 233, a step current circuit 239, an analog-to-digital converter(ADC) 235 and a digital back end 237, wherein the capacitive touch panel21, the attenuation circuit 233, the step current circuit 239 and theADC 239 are coupled to the IPGA 231. The attenuation circuit 233 is usedto attenuate a gain of the IPGA 231 by drawing a part of current. TheADC 235 is coupled between the IPGA 231 and the digital back end 237.

In this embodiment, the calibration stage is for storing the calibrationparameter used in the normal stage, e.g., control codes for controllinga plurality of switching devices in the step current circuit 239,instead of performing the touch detection. The normal stage is forperforming the touch detection according to the stored calibrationparameter determined in the calibration stage.

Step S61: Firstly, during the starting procedure or at the end of thesleep mode, the touch device automatically performs the calibrationmode. Or the digital back end 237 controls the touch device to enter thecalibration stage after receiving an instruction signal, e.g., when akey is pressed or an APP is run.

Step S63: After the calibration stage is entered, the plurality of drivecircuits 230 of the control chip 23 respectively inputs a predeterminedconstant voltage to the corresponding drive electrode 211 or does notinput any signal to the corresponding drive electrode 211. Each of theIPGAs 231 receives the detection signal y(t) from the correspondingsense electrode 213 and performs the amplification according to a gaindetermined by the element parameter (e.g., the feedback resistor Rf,compensation capacitor Cf and input resistor Ri) to output an amplifieddetection signal Sout.

Step S65: The ADC 235 performs the analog-digital conversion on theamplified detection signal Sout outputted by each of the IPGAs 231 tooutput a digital signal corresponding to each of the IPGAs 231.

Step S67: The digital back end 237 receives the digital signal (e.g., adigital value), and compares the received digital signal withpredetermined digital value. As mentioned above, the digital back end237 preferably pre-stores predetermined digital values corresponding toeach of the IPGAs 231. For example, when the received digital signal islarger or smaller than the predetermined digital value, the switchingdevices 0 to 2 are controlled to provide different step currents to theIPGA 231 or the switching devices 3 to 5 are controlled to drawdifferent step currents from the IPGA 231 till the offset current formedby the offset voltage difference between the IPGA 231 and theattenuation circuit 233 has a smallest value, e.g., controlling theswitching devices one-by-one in a predetermined sequence to obtain thesmallest value. In this embodiment, the controlling of the switchingdevices 0 to 5 is formed as digital codes having a predetermined bit tobe stored as the control codes.

When the control codes corresponding to every IPGA 231 are stored in thestorage device of the digital back end 327, the calibration stage isended.

Step S69: After the calibration stage is over, the normal stage orso-called operation mode is automatically entered. The control chip 23controls the multiple drive circuits 230 therein to respectively input,sequentially or concurrently, a drive signal Vsig to the correspondingdrive electrode 211, wherein the drive signal Vsig is acontinuous/non-continuous signal and periodic/non-periodic signalwithout particular limitations. Details of the concurrently driving maybe referred to U.S. patent application Ser. No. 13/928,105, filed onJun. 26, 2013, assigned to the same Assignee of the present application,and the full disclosure of which is incorporated herein by reference.Meanwhile, corresponding to a scan signal, which controls the sequentialreading of each of the multiple sense electrodes 213, the digital backend 237 of the control chip 23 reads the pre-stored control codecorresponding to the sense electrode 213 currently being scanned fromthe storage device to control the corresponding switching devices (e.g.,0 to 5 in FIG. 5) in the step current circuit 239 thereby reducing theoffset current caused by the offset voltage difference between the IPGA231 and the attenuation circuit 233.

Accordingly, it is able to effectively alleviate the operable dynamicrange decrease caused by the offset voltage thereby increasing thesignal-to-noise ratio and the detection performance.

It is appreciated that the values in the above embodiment such as theresistance, capacitance, voltage value, current value, number ofswitching devices and so on, are only intended to illustrate but not tolimit the present disclosure.

As mentioned above, the signal processing circuit of a conventionaltouch panel has a lower dynamic range due to the offset voltage of anoperational amplifier therein such that it has lower SNR and touchcontrol performance. Therefore, the present disclosure further providesa touch device (as shown in FIG. 2) and a signal processing circuit (asshown in FIGS. 4-5) as well as an operating method (as shown in FIG. 6)thereof that may eliminate the influence caused by the offset voltage ofan operational amplifier by adopting a step current circuit to provideor draw a step current to cancel out or reduce at least a part of thecurrent flowing through the feedback resistor of the operationalamplifier.

Although the disclosure has been explained in relation to its preferredembodiment, it is not used to limit the disclosure. It is to beunderstood that many other possible modifications and variations can bemade by those skilled in the art without departing from the spirit andscope of the disclosure as hereinafter claimed.

What is claimed is:
 1. A signal processing circuit of a touch device,the signal processing circuit comprising: an integrated programmablegain amplifier (IPGA), comprising: a first operational amplifier havinga positive input, a negative input and an output terminal; a feedbackresistor connecting between the negative input and the output terminalof the first operational amplifier; a compensation capacitor connectingbetween the negative input and the output terminal of the firstoperational amplifier; and an input resistor, a first terminal of theinput resistor coupled to the negative input of the first operationalamplifier; an attenuation circuit coupled to a second terminal of theinput resistor to split a current flowing through the input resistor; astep current circuit, coupled to the negative input of the firstoperational amplifier, and configured to provide a step current to theIPGA or draw a step current from the IPGA, the step current circuitcomprising a current mirror circuit which comprises a plurality ofswitching devices; and a digital back end configured to store controlcodes for controlling on/off of the plurality of switching devices todetermine the step current during a touch detection.
 2. The signalprocessing circuit as claimed in claim 1, wherein the current flowingthrough the input resistor is an offset current formed by an offsetvoltage difference between the IPGA and the attenuation circuit.
 3. Thesignal processing circuit as claimed in claim 1, wherein the digitalback end is configured to store the control codes in a calibration stageto cause a current flowing through the feedback resistor to have asmallest value, and control, in a normal stage for the touch detection,the on/off of the plurality of switching devices according to the storedcontrol codes.
 4. The signal processing circuit as claimed in claim 1,wherein the attenuation circuit comprises: a second operationalamplifier having a positive input, a negative input and an outputterminal, and the output terminal of the second operational amplifierbeing coupled to the negative input of the second operational amplifier;and a shunt resistor having a first terminal being coupled to the outputterminal of the second operational amplifier, and a second terminalbeing coupled to the second terminal of the input resistor.
 5. A touchdevice, comprising: a capacitive touch panel comprising a plurality ofsense electrodes each configured to output a detection signal; a controlchip, comprising: a plurality of integrated programmable gain amplifiers(IPGAs) respectively coupled to the plurality of sense electrodes toamplify the detection signal, each of the IPGAs comprising: a firstoperational amplifier having a positive input, a negative input and anoutput terminal; a feedback resistor connecting between the negativeinput and the output terminal of the first operational amplifier; acompensation capacitor connecting between the negative input and theoutput terminal of the first operational amplifier; and an inputresistor, a first terminal of the input resistor coupled to the negativeinput of the first operational amplifier; a plurality of attenuationcircuits each coupled to a second terminal of the input resistor of oneof the IPGAs, and configured to split a current flowing through theinput resistor; a plurality of step current circuits each coupled to thenegative input of the first operational amplifier of one of the IPGAs,and configured to provide a step current to the coupled IPGA or draw astep current from the coupled IPGA, each of the plurality of stepcurrent circuits comprising a current mirror circuit which comprises aplurality of switching devices; and a digital back end configured tostore control codes for controlling on/off of the plurality of switchingdevices of each of the plurality of step current circuits to determinethe step current to be provided to or drawn from the coupled IPGA duringa touch detection.
 6. The touch device as claimed in claim 5, whereinthe control chip further comprises: an analog-to-digital converter,coupled to the output terminal of the first operational amplifier ofeach of the IPGAs, and configured to convert the amplified detectionsignal to a digital signal, wherein the digital back end is coupled tothe analog-to-digital converter.
 7. The touch device as claimed in claim6, wherein the digital back end is configured to store the control codesin a calibration stage to cause a current flowing through the feedbackresistor to have a smallest value, and control, in a normal stage forthe touch detection, the on/off of the plurality of switching devicesaccording to the stored control codes.
 8. The touch device as claimed inclaim 7, wherein the control chip further comprises a plurality of drivecircuits configured to output drive signals to the capacitive touchpanel in the normal stage, and not output the drive signals to thecapacitive touch panel in the calibration stage.
 9. The touch device asclaimed in claim 5, wherein the control chip further comprises a storagedevice configured to store the control codes respectively correspondingto each of the plurality of IPGAs.
 10. The touch device as claimed inclaim 5, wherein each of the attenuation circuit comprises: a secondoperational amplifier having a positive input, a negative input and anoutput terminal, and the output terminal of the second operationalamplifier being coupled to the negative input of the second operationalamplifier; and a shunt resistor having a first terminal being coupled tothe output terminal of the second operational amplifier, and a secondterminal being coupled to the second terminal of the input resistor. 11.An operating method of a touch device, the touch device comprising acapacitive touch panel, an integrated programmable gain amplifier(IPGA), an attenuation circuit, a step current circuit, ananalog-to-digital converter and a digital back end, the capacitive touchpanel, the attenuation circuit, the step current circuit and theanalog-to-digital converter being coupled to the IPGA, the attenuationcircuit attenuating a gain of the IPGA, the step current circuitcomprising a current mirror circuit which comprises a plurality ofswitching devices, and the analog-to-digital converter being coupledbetween the IPGA and the digital back end, the operating methodcomprising: entering a calibration stage; receiving, by the IPGA in thecalibration stage, a detection signal outputted by the capacitive touchpanel to output an amplified detection signal; converting, by theanalog-to-digital converter in the calibration stage, the amplifieddetection signal to a digital signal; controlling, by the digital backend in the calibration stage, on/off of the plurality of switchingdevices of the step current circuit to provide a step current to theIPGA or draw a step current from the IPGA according to the digitalsignal to reduce an offset current value caused by an offset voltagedifference between the IPGA and the attenuation circuit; and storing acontrol code for controlling the on/off of the plurality of switchingdevices to cause the offset current value to have a smallest value. 12.The operating method as claimed in claim 11, further comprising:entering a normal stage; and controlling, in the normal stage, theplurality of switching devices of the current mirror circuit accordingto the stored control code.
 13. The operating method as claimed in claim11, wherein the IPGA comprises: a first operational amplifier having apositive input, a negative input and an output terminal; a feedbackresistor connecting between the negative input and the output terminalof the first operational amplifier; a compensation capacitor connectingbetween the negative input and the output terminal of the firstoperational amplifier; and an input resistor, a first terminal of theinput resistor being coupled to the negative input of the firstoperational amplifier, and a second terminal of the input resistor beingcoupled to the attenuation circuit.
 14. The operating method as claimedin claim 11, wherein the attenuation circuit comprises: a secondoperational amplifier having a positive input, a negative input and anoutput terminal, and the output terminal of the second operationalamplifier being coupled to the negative input of the second operationalamplifier; and a shunt resistor having a first terminal coupled to theoutput terminal of the second operational amplifier, and a secondterminal coupled to the IPGA.